Method and apparatus for performing variable word width searches in a content addressable memory

ABSTRACT

A content Addressable memory (CAM) for performing search operations using variable width search data, said CAM comprising a plurality of arrays of CAM cells, each coupled to a respective sub-search data bus, the sub-search buses being confined to form a main search data bus, to which is applied the search data; selector circuits receiving match line signals from respective CAM arrays, the match line signals being indicative of the results of a search and comparison formed in the associated CAM array, the selector circuit being responsive to a mode selection signal for selecting one or more of said match line output signals to be switched to a priority encoder and multiple match resolver (PE-MMR), wherein in a first mode the match line output signals are switched to said PE-MMR and in a second mode groups of match line output signals from selected arrays are switched to said PE-MMR.

This application is a continuation application from U.S. patentapplication Ser. No. 10/902,687, filed on Jul. 30, 2004, which is acontinuation application from U.S. application Ser. No. 10/158,196,filed May 31, 2002, now U.S. Pat. No. 6,771,525.

The present invention relates to a Content Addressable Memory (CAM), andmore specifically to a method and circuit for searching a CAM withvariable width search data, wherein the width of the search data may bechanged on each search instruction.

BACKGROUND OF THE INVENTION

A CAM is a memory device which allows a processor within a computer, toperform a parallel search through the CAM's stored data in order toretrieve the address of any matching stored data if such data is found.The search and stored data can be either binary (comprising logic “1” or“0” state) or ternary (logic “1”, “0” or “don't care” states) data.

A CAM is usually subdivided in banks of CAM cell arrays. A CAM arraycomprises a plurality of CAM cells arranged in rows and columns with rowof the cell being connected to an associated match line and cells beingconnected to associated search lines. The cells are also coupled towordlines and bitlines for retrieving information out of and for storinginformation in the CAM.

During a search operation, search data often referred to as a search keyof a particular width is placed on the search lines. The search key onthe search data lines is compared with stored data in all CAM celllocations simultaneously and a match (also referred to as a “hit”) ormismatch result (also referred to as a “miss”) is provided to matchlines coupled to each CAM cell. If the search key matches theinformation stored in a particular row in the CAM, the associated matchline will indicate a hit or match condition. If the search key and thestored data are different, the associated match line will indicate amiss or mismatch condition. All match line outputs are processed by amultiple match resolver (MMR) and priority encoder (PE) block that willprovide as an output signal the highest priority match address where thematch information is located.

Conventional CAM's are mode configurable to perform search operations ondata words having specific word widths for example, 144 bit or 288 bitsearch modes. Typically in such CAM's, a configuration register storesthe operating mode of the device and fields within the register specifythe word width. Data will be written into the CAM according to the wordwidth setting in the mode register and search instructions issued to theCAM will therefore search the stored data based on the word widthspecified in the configuration register. In order to change the wordwidth the mode register must first be updated and then the new search isperformed. Thus, variable word width searching “on-the-fly” is notsupportable by such conventional CAM's. Thus, although pre-setword-width CAM's do exist, there is still a need for a CAM that supportsarbitrary word width searches and which is capable of switching betweenone or more types of searching on-the-fly.

Some conventional approaches have been proposed for searching wordswhich are wider than default word widths stored in a CAM, for example inU.S. Pat. No. 6,252,789 to Pereira et al. Such approaches howeverrequire extra width expansion circuitry associated with each CAM row forhandling situations when search data words that are wider than thedefault stored word width. Furthermore, such width expansion circuitryare interconnected and a match result is not provided to a priorityencoder until the last word in a multiword data chain has been comparedto the contents of the data stored in the CAM. This approach thereforeis only effective in exact match multiple word searches.

Thus, there is still a need for a CAM that is capable of performingvariable word width searching with search data widths that can bechanged on-the-fly, and while storage of data words makes efficient useof the CAM.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a contentaddressable memory (CAM) for performing search operations using variablewidth search data, the CAM comprising:

-   a) a plurality of arrays of CAM cells, each coupled to a respective    sub-search data bus, the sub-search buses being confined to form a    main search data bus, to which is applied the search data;-   b) selector circuits receiving match line signals from respective    CAM arrays, the match line signals being indicative of the results    of a search and comparison formed in the associated CAM array, the    selector circuit being responsive to a mode selection signal for    selecting one or more of the match line output signals to be    switched to a priority encoder and multiple match resolver (PE-MMR),    wherein a first mode or all match line output signals are switched    to the PE-MMR and in a second mode groups of match line output    signals from selected arrays are switched to the PE-MMR, whereby    wide width data searching is performed by simultaneously evaluating    match line outputs from the plurality of arrays.

An advantage of the invention is the ability to perform variable wordwidth searches (72, 144 and 288 bit wide words for example) with theability to change the searchable word width mode on-the-fly; i.e. oneach new search instruction issued to the CAM device.

In accordance with another aspect of the invention, there is provided amethod for performing variable word width searching in a contentaddressable memory comprises selectively combining match line latchoutputs from two adjacent CAM memory arrays and providing theselectively combined match line outputs to a multiple matchresolver/priority encoder in response to a variable word width controlsignal. If a 72 bit mode search is performed, the priority encoder willsimultaneously receive and process four separate match line latchoutputs match line locations in adjacent arrays. If a 144 bit modesearch is performed, then a combination of two match line latch outputsare provided from two adjacent arrays to the priority encoder. If a 288bit mode search is performed, a combination of four match line latchoutputs is provided to the priority encoder.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention are illustrated byway of example and are by no means intended to limit the scope of thepresent invention, the particular embodiments of which are described inthe following figures as follows:

FIG. 1 is a simplified block diagram of a CAM according to an embodimentof the invention;

FIG. 2 is a simplified circuit and block diagram illustrating anembodiment of the invention;

FIG. 3 is a schematic diagram showing the implementation of a word widthselection circuit;

FIG. 4 is a timing diagram showing a 72 bit wide search operationaccording to the present invention;

FIG. 5 is a timing diagram showing a 144 bit wide search operationaccording to the present invention; and

FIG. 6 is a timing diagram showing a 288 bit wide search operationaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description like numerals refer to like structures inthe drawings.

FIG. 1 illustrates the general structure of a CAM 100 according to anembodiment of the invention. The CAM 100 comprises two arrays 102 and104 respectively, wherein each CAM array is comprised of 512 rows, andassociated match lines ML 108, by 72 bits (not including any redundancyrows or columns). The plurality of arrays of CAM cells, each coupled toa respective sub-search data bus, the sub-search buses being combined toform a main search data bus, to which is applied the search data. Inthis example each row of memory cells is connected to a single matchline ML 108 and can store up to 72 bits of data, which can be binarydata or ternary data. In order to store words wider than 72 bits, twoarrays are used to store an upper and lower portion the a wide word. Forexample, in order to store a 144 bit word, 72 bits would be stored inone array in a row and the other 72 bits would be stored in anassociated row in the adjacent array. In order to store a 288 bit word,two 144 bit words would be stored as described at two subsequent rowlocations in the two adjacent arrays.

Each match line ML 108 has an associated match line latch LTCH 110 forstoring the result of a search performed on that row. The match linelatch LTCH 110 has a 1 bit match line latch output LTCHO 112 which willbe set to logic “1”, representing a “hit” condition, if the searcheddata matches the stored data, or will be set to a logic “0”,representing a “miss” condition, if the search data and stored data donot match. Each of the match line latch output LTCHO 112 is connected tothe priority encoder and multiple match resolver 106. If a plurality of“hits” are found in a CAM array, the priority encoder and multiple matchresolver 106 will choose the hit having the highest priority address andprovide that highest priority address as the output 118. For example,with reference to FIG. 1, a representative group of four match lines andtheir corresponding match line latch outputs LTCHO 112 are designated byfour outputs A₀, B₀, C₀ and D₀. This pattern is repeated for theremainder of the arrays as A_(i), B_(i), C_(i) and D_(i) wherein i takesvalues from 0 to N.

In order to support variable word width searches, the search data bus114, shown as 144 bits wide, is split into two 72 bit wide buses eachaccessing arrays 102 and 104 respectively. When a search is performed,all the CAM arrays are accessed in parallel. Along with search dataprovided on the search data bus 114, a search mode control signalindicative of the word width search mode must be provided. The searchmode control signal will indicate whether a 72 bit, 144 bit or 288 bitsearch is to be performed. By providing this control signal with everysearch instruction, the device has the capability to perform differentword width searches with each consecutive search instruction, i.e., thedevice can support variable word width on-the-fly searching.

If a 72 bit search instruction is issued, the same 72 bit search keywill be loaded onto the upper 72 bits and the lower 72 bits of thesearch data bus 114 and the search data will be compared with storeddata in all rows of the two CAM arrays. A match may result in any one ormore 72 bit locations in either of the two arrays.

If a 144 bit search instruction is issued, the 144 bit search data willbe loaded onto the search data bus 114 and will be compared with storeddata in the two CAM arrays 102 and 104. A match condition will occur iffor any particular row, there will be a match of the upper and lower 72bits of the search word.

If a 288 bit search instruction is issued, then two consecutive searchesof 144 bits each are performed. A match result requires that fouradjacent 72 bit segments match the search data, two in array 102 and twoin array 104.

FIG. 2 shows a preferred embodiment of the present invention supporting72, 144 or 288 bit word width searching, and providing the ability tochange search modes “on-the-fly”. The match line latch outputs LTCHO 112are designated as A₀, B₀, C₀, and D₀ for the first group of fourmatchline latch outputs, each representing the result of a 72 bit searchin every matchline 108.

If a 72 bit search is to be performed on the CAM, the user (processor)will provide the search mode information to the search mode selectioncircuit 130. The search mode selection circuit 130 comprises modeselector circuitry 132 and a circuit for selectively combining thematchline latch outputs LTCHO 112 comprising two NAND logic gates 138,140, a logic NOR gate 142, and inverters 144 and 146.

When a 72 bit search is performed, each latch LTCH 110 in everymatchline 108 of both CAM arrays 102 and 104 are on an active statelatching the result. This is achieved by asserting latch enable signalsLTEN 1, LTEN 2. This is achieved by asserting latch enable signals LTEN1, LTEN 2. That is latch signal enable LTEN 1 is applied for the first144 bits and LTEN2 is asserted for the second 144 bits. Latch outputsignals LTCHO 112 A₀, B₀, C₀ and D₀ are provided through the modeselector 132 directly to the priority encoder 106 through the 4 bit wideoutput bus 136. This occurs when the select signal SEL1 is in a highlogic state while signal SEL1/ will be in a low logic state and signalsSEL2 and SEL2/ are in “don't care” logic state. Since the search is of72 bit mode, none of the results provided by the logic gates 138, 140and 142 are selected by the mode selector circuitry 132.

When a 144 bit search is to be performed, this instruction is providedto the search mode selection circuitry 130. Also, the 144 bit search keyis divided into two 72 bit search keys, with the first 72 bits beingprovided to the first CAM array 102 and the second 72 bits beingprovided to the second CAM array 104. Once the search and compareoperation is performed, all the latches LTCH 110 are activated latchingthe matchline result of the comparison. The latch output results LTCHO112 A₀, B₀, C₀ and D₀ are combined in NAND gates 138 and 140. Theoutputs of NAND gates 138 and 140 are provided to inverters 144 and 146resulting in signals A₀B₀ and C₀D₀ respectively, which carry the resultof a 144 bit search. The mode selector 132 receives the signals SEL1 setto logic low and signal SEL1/ set to logic high, thereby passing a“wide” search result to the priority encoder and multiple match resolver106. In this case, the selection signal SEL2 is set to logic high andsignal SEL2/ is set to logic low for providing the 144 bit searchresult. It should be noted that although the 72 bit search results A₀,B₀, C₀ and D₀ are received by the mode selector circuit 132, they arenot provided to the priority encoder and multiple match resolver 106.

If a 288 bit search is chosen, the search is performed in two searchcycles, each search cycle following a similar procedure as a 144 bitsearch followed for another 144 bit search, with some differences. Inthis case, the 288 bit key search is divided into four 72 bit searchkeys. The first and second 72 bit key words are submitted to the CAMarrays 102 and 104 respectively as in a 144 bit search. Then the latchesLTCH 110 are selectively activated so that latches corresponding tomatchlines A₀ and B₀ latch the result of the first search cycle.Thereafter, a second search with the third and fourth 72 bit key wordsare submitted to the CAM arrays 102 and 104 as in the previous step, butonly the latches LTCH 110 corresponding to matchlines C₀ and D₀ areactivated. Once all the results of the 288 bit search are latched as A₀,B₀, C₀ and D₀, they are provided to the NAND logic gates 138, 140,generating signals AB₀ and CD₀ respectively which are further combinedby NOR gate 142, generating the the ABCD₀ signal. Mode selector 132receives selector signals SELL and SEL2 set to logic low (causingsignals SEL1/ and SEL2/ to be set to logic high) and as a result, signalABCD₀ is provided to the priority encoder and multiple match resolver106.

In this manner a user can issue consecutive search instructions ofvarying word width, including 72, 144 or 288 bit wide searches using thesame Content Addressable Memory.

FIG. 3 is a detailed schematic illustrating the circuitry containedwithin the mode selector circuit 132 of FIG. 2. As can be seen, the modeselector circuit 132 is a multiplexer that receives as inputs thesignals A_(i),B_(i),C_(i),D_(i), AB_(i), CD_(i), ABCD_(i), as providedby the logic gates discussed with reference to FIG. 2. The multiplexer132 is controlled by the select signals SEL1, SEL1/, SEL2, SEL2/ andoutputs a 4 bit wide output signal 136 which is provided to the priorityencoder/multiple match resolver circuit 106 as shown in FIG. 2. Theoperation of the multiplexer 132 will be readily apparent to someoneskilled in the art and will therefore not be discussed in furtherdetail.

FIGS. 4, 5 and 6 illustrate simplified relative timing signalscorresponding to the 72, 144 and 288 bit wide search operations.

On FIG. 4, the user (i.e. processor) provides a 72 bit search data S1 toboth CAM arrays 102 and 104. All latches for matchlines A, B, C and Dare latched and the result R1 is provided to the priority encoder andmultiple match resolver 106. Subsequent 72 bit searches can be submittedin the same manner, providing respective results R2, R3, R4 as shown.

FIG. 5 corresponds to a 144 bit search operation and is similar to FIG.4 with the difference that the search key provided is 144 bit wide andthe output 144 bit wide output R1 represents the combination of signalsAB or CD respectively.

FIG. 6 corresponds to a 288 bit wide search operation and differs fromthe previous two timing diagrams in the sense that the search data isprovided in two steps, shown as S1 a for the first 144 bit search keyand S1 b for the second 144 bit search key. During the first searchcycle, latches A and B are activated and their result remains unchangedthrough the following search cycle when the second search is performed(S1 b) and latches C and D are activated. Once all four signalscomprising the result of the 288 bit search are available, the ABCDsignal is delivered to the priority encoder and multiple match resolver106 as R1.

A further advantage of the present invention which can be obtained whileperforming 288 bit searches is that the user can retain a partial 144bit search result and combine it with another 144 bit search key andobtain the 288 bit search result in the subsequent clock cycle. Forexample, if a first 288 bit search word is provided to the CAM as twosequentially submitted 144 bit words, the result of the 288 bit searchwill be available for the priority encoder/multiple match resolver 106in two clock cycles as discussed with reference to FIG. 6. The searchwill comprise two consecutive 144 bit searches. According to thepreferred embodiment of the invention described above, the CAM willselectively latch the first 144 bit search result (AB) in the firstclock cycle and then selectively latch the second 144 bit search result(CD) in the second clock cycle. Since both search results (AB and CD)are stored in their respective latches, during a third clock cycle, if anew 288 bit search is issued with search key ABC′D′ for example, theoutputs AB from the first clock cycle can be combined with the new 144bit search of C′D′ to yield a new intermediate 288 bit search ABC′D′result without having to re-search the entire 288 bits. Thus we needonly assert LTEN 2. Consequently, the user can change any 144 bitportion of a 288 bit search word and obtain the result of the new 288bit search in one subsequent clock cycle.

This type of variable word width searching therefore allows large wordwidth searches to take place without having to re-search previous bits,which have already been searched. This allows the user to change only acertain portion of the wide word and re-do the search rather than havingto re-search the entire wide word. Furthermore, this structure does notrequire wrap-around storage of wide words, i.e. storage of words thatare wider than the width of an array and are continued or “wrappedaround” in a subsequent row of the same array.

It may be noted that the timing diagrams as shown are intended to showthe sequence of signals on the various inputs, however the relativetiming between signals will vary depending on the particular CAM designbeing used. Thus a person skilled in the art would recognize that anormal latency comprising some clock cycles, is included between thenormal operation of a CAM like this, which means that the address thatis retrieved by the user will not be available directly in the“following clock cycle”. If the CAM comprises features as segmentedmatchlines, presearch and main search, cascaded features, etc. the delaytime is increased for the overall normal operation of the chip.

Although the invention has been described with reference to specificembodiments, various modifications will become apparent to a personskilled in the art without departing from the spirit of the invention.

1. A variable word width searchable content addressable memorycomprising: an array of content addressable memory cells, each row ofcontent addressable memory cells storing one of a wide word of data andat least one word of data; and, at least one search data bus coupled toeach row of content addressable memory cells for providing word searchdata to the at least one word of data and for providing wide word searchdata to the wide word of data.